As primary care physicians, we advise patients to quit smoking. However, BMI gain often follows smoking cessation, and managing the potential negative impacts of BMI gain also falls within our clinical responsibilities. Meanwhile, Alzheimer's dementia and vascular dementia are growing disease entities that are major concerns among elderly patients. It has not been clarified whether BMI changes following smoking cessation influence the risks of Alzheimer's or vascular dementia. Therefore, we conducted a retrospective cohort study to investigate this association.To clarify the association between BMI changes following smoking cessation and the risks of Alzheimer’s dementia and vascular dementiaWe performed a retrospective cohort study using a nationwide health screening database that included 224,588 adults aged 40 years or older who underwent health screenings between 2009 and 2012. Smoking behaviors were self-reported, and BMI changes were calculated over time. Our primary outcomes were overall dementia, Alzheimer’s dementia, and vascular dementia, which were tracked until the end of 2019. Primary outcomes were defined using the International Classification of Diseases codes along with outpatient clinic and hospital admission records.Never smokers showed a significantly lower risk of dementia compared to continual smokers (aHR 0.85, 95% CI 0.73-0.97). Compared to quitters with minimal BMI change (net BMI change < 1.0), quitters with BMI gain (BMI change ≥ +1.0) showed a reduced Alzheimer’s dementia risk (aHR 0.52, 95% CI 0.29-0.93). Further stratifying BMI gain (+1.0, +2.0, +3.0 or more) did not change this trend of reduced dementia risk.BMI changes did not significantly alter the general beneficial impacts of smoking cessation on dementia risks. Also, when compared among quitters, BMI gain was associated with further decreasing dementia risks, and this pattern remained consistent even with substantial BMI gain (≥+3.0). This aligns with previous literature that has demonstrated protective effects of late life BMI gain against dementia.Primary care physicians should confidently advise patients that smoking cessation reduces dementia risk, and that BMI gain may even provide further benefits, especially for individuals aged 65 years or older.